This invention relates to a trouble warning device which gives warning of trouble in an air-fuel ratio sensor mounted in an internal combustion engine.
There is known an air-fuel ratio feedback control system for controlling the air-fuel ratio (A/F ratio) in the exhaust gas of an internal combustion engine so that it is a stoichiometric A/F ratio. In this system, an air-fuel ratio sensor (A/F ratio sensor), for example, an oxygen concentration sensor is included in an exhaust system of the engine to detect the A/F ratio in the exhaust gas. A sensed signal from the A/F ratio sensor is fed to an electronic fuel injection control means to correct the amount of fuel injected into the engine, or fed to a secondary air injection control means to control the amount of secondary air injected into the engine so as to effect the above A/F ratio control. If a three way catalytic converter capable of simultaneously eliminating three harmful components, i.e., HC, CO and NO.sub.x, were to be combined with this air-fuel ratio feedback control system, it would seem to be possible to obtain a purified exhaust gas. This is because the operation of this three way catalytic converter is effectively attained when the A/F ratio of the exhaust gas introduced into the converter is kept near the stoichiometric A/F ratio.
An A/F ratio sensor used in the above-mentioned system, to which a constant current should be provided, generally generates an output voltage of about 1 V when the A/F ratio in the atmosphere of the sensor is smaller than the stoichiometric A/F ratio, namely when the atmosphere is rich, and; it generally generates an output voltage of about 0.1 through 0.2 V when the A/F ratio in the atmosphere of the sensor is greater than the stoichiometric A/F ratio, namely when the atmosphere is lean. However, when the A/F ratio sensor malfunctions, the level of the output voltage thereof becomes approximately as high as the level of a source voltage provided from a voltage supply, for example, a battery which is mounted in the engine.
In a known system for giving warning of trouble in the A/F ratio sensor, a comparator disposed in the electronic fuel injection control means, or in the secondary air injection control means, so as to control the A/F ratio of the exhaust gas has been used to also detect trouble in the A/F ratio sensor. However, according to this known system, since the reference voltage level of the comparator is low (about 0.5 V), it is difficult to discriminate whether trouble in the A/F ratio sensor has occurred or the atmosphere of the A/F ratio sensor has become rich. Therefore, an error in detection of trouble in the A/F ratio sensor has often occurred in the known system.